Good question

`svn update` works on a "working copy" of a repository, which is the code you've checked out with `svn checkout` -- it's got a whole bunch of those `.svn` folders if you look closely, which store information about your working copy, revisions, repository info, and a lot more. Update is used to refresh that with the latest from the repository.
`svn export` is completely different, and you shouldn't confuse the two. Export just gets all the files from the repository [i]without any SVN metadata attached[/i]. What you get from export is not a working copy, and can't be checked back in if you make changes, but it is a fresh clean copy of the files in your repo, usually for release or actual usage (because you don't want .svn folders all over the place in your production folders -- they take up space and aren't needed at all).
So that's the difference. Feel free to ask more questions :-)